Proportional to temperature voltage generator

ABSTRACT

A biasing circuit comprising a first circuit and a second circuit. The first circuit may be configured to generate a first bias signal and a second bias signal. The second bias signal may be defined by a threshold voltage and a first resistance. The second circuit may be configured to generate a third bias signal in response to the first and the second bias signals and a second resistance. The third bias signal may have a magnitude that is linearly proportional to absolute temperature (PTAT) and be configured to vary a refresh rate of a memory cell in response to changes in temperature.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and/or architecture forvoltage generators generally and, more particularly, to a method and/orarchitecture for a proportional to absolute temperature (PTAT) voltagegenerator.

BACKGROUND OF THE INVENTION

[0002] Data (e.g., a “1” or a “0”) is stored in a 1T memory cell as avoltage level. A “1” is stored as a high voltage level which candecrease due to leakage. A “0” is stored as a voltage level of zerovolts which can increase due to leakage. The 1T memory cell requires aperiodic refresh to maintain the voltage level stored in the cell. Inmany applications, a memory chip uses a ring oscillator to control whenthe refreshes occur. The frequency of a signal generated by a typicalring oscillator decreases with increasing temperature because of CMOSdevice characteristics. However, the memory cell leakage increases withtemperature. As the temperature increases, refresh using a conventionaloscillator can occur less frequently than necessary to maintain thevoltage level stored in the memory cell. Thus, the oscillator needs tobe designed to support the high temperature refresh rate at the expenseof more current.

[0003] Proportional to absolute temperature(PTAT) voltages and currentsare used in temperature monitoring circuits. The monitoring circuitseither detect a specific temperature or output a voltage and/or currentthat increases with temperature. The temperature monitoring circuits canuse a PTAT and an inverse PTAT, where the crossing point is a desiredtemperature. A conventional method of generating PTAT voltage is to usea delta Vbe generator circuit.

[0004] Referring to FIG. 1, a block diagram of a circuit 10 is shown.The circuit 10 is a delta Vbe generator circuit that can generate a PTATvoltage VREF. The voltage VREF is described by the following equation 1:$\begin{matrix}{{Vref} = {{{Vbe}\quad 1} = {\frac{n \cdot k}{q} \cdot {\ln \left( {\frac{n \cdot k \cdot {\ln (B)} \cdot T}{q \cdot A \cdot {Is} \cdot R} + 1} \right)} \cdot T}}} & {{Eq}.\quad 1}\end{matrix}$

[0005] where T is the absolute temperature in Kelvin, n is the emissioncoefficient, k is Boltzmann's constant, q is the charge of an electron,Is is the theoretical reverse saturation current, A is the smaller ofthe areas of diodes 12 and 14, B is the ratio of the areas of the diodes12 and 14, and R is the resistance of the resistor 16. The resistance Rgenerally has a positive temperature coefficient. The emissioncoefficient n is related to the doping profile and affects theexponential behavior of the diodes 12 and 14. The value of n is normallyapproximated to be 1.

[0006] The voltage VREF is proportional to the temperature T, ln(T), and1/R(T). Also, a current I is generated equal to Vt*ln(B)/R which isproportional to temperature since R has a positive temperaturecoefficient and Vt=k*T/q. The voltage VREF is generated by using avoltage across a diode with the bandgap current I flowing through thediode. The circuit 10 has the following disadvantages: a complexrelationship between temperature and the voltage VREF (i.e., the voltageVREF is a function of T, ln(T), and ln(1/R(T)); the value of the voltageVREF is limited when the bandgap current I is also used to generate aPVT compensated voltage; and a larger value for the voltage VREFrequires a higher current I.

SUMMARY OF THE INVENTION

[0007] The present invention concerns a biasing circuit comprising afirst circuit and a second circuit. The first circuit may be configuredto generate a first bias signal and a second bias signal. The secondbias signal may be defined by a threshold voltage and a firstresistance. The second circuit may be configured to generate a thirdbias signal in response to the first and the second bias signals and asecond resistance. The third bias signal may have a magnitude that islinearly proportional to absolute temperature (PTAT) and be configuredto vary a refresh rate of a memory cell in response to changes intemperature.

[0008] The objects, features and advantages of the present inventioninclude providing a method and/or architecture for a proportional toabsolute temperature (PTAT) voltage generator that may (i) use a bandgapreference with a current equal to Vt*ln(B)/R, (ii) use one additionalresistor to form a linear PTAT voltage reference, and/or (iii) provide aPTAT voltage reference that may be scaled by a ratio of resistor values.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andthe appended claims and drawings in which:

[0010]FIG. 1 is a block diagram of a delta Vbe generator circuit;

[0011]FIG. 2 is a block diagram of a preferred embodiment of the presentinvention;

[0012]FIG. 3 is a block diagram of an implementation of the presentinvention; and

[0013]FIG. 4 is a block diagram of a memory device in accordance with apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Referring to FIG. 2, a block diagram of a circuit 100 is shown inaccordance with a preferred embodiment of the present invention. Thecircuit 100 may be implemented as a proportional to temperature voltagegenerator circuit. The circuit 100 may be configured to generate a firstvoltage signal (e.g., NCTR) and a second voltage signal (e.g., PCTR)that may be proportional to absolute temperature (PTAT). The circuit 100may comprise a circuit 102 and a circuit 104. The circuit 102 may beimplemented as a PTAT current source circuit. The circuit 104 may beimplemented as a PTAT voltage reference circuit. The circuit 102 may beconfigured to generate a temperature dependent reference signal (e.g.,VREF) and a bias signal (e.g., VBIAS). The signal VREF may vary linearlywith temperature. The signal VREF may be presented to an input 106 ofthe circuit 104. The signal VBIAS may be presented to an input 108 ofthe circuit 104. The circuit 104 may be configured to generate thesignals NCTR and PCTR in response to the signal VREF and the signalVBIAS. The signal PCTR may be a mirror of the signal NCTR.

[0015] The circuit 102 may comprise a transistor 110, a transistor 112,a transistor 114, a transistor 116, a transistor 118, a device 120, adevice 122, a device 124, and an amplifier 126. The transistors 110-114may be implemented as one or more PMOS transistors. The transistors 116and 118 may be implemented as one or more NMOS transistors. However,other types and/or polarity of transistors may be implementedaccordingly to meet the design criteria of a particular application. Thedevices 120 and 122 may be implemented as base-emitter junction devices(e.g., diodes, diode-connected transistors, etc.). In one example, thedevices 120 and 122 may be implemented as forward biased diodes. Thedevice 120 may have an area A. The device 122 generally has an area thatis B times A, where B is an integer. The device 124 may be implementedas a resistive circuit. In one example, the device 124 may beimplemented as a resistor having a predetermined resistance R. Theamplifier 126 may be implemented as an operational amplifier circuit.

[0016] The transistors 112-118 and the devices 120-124 may be configuredas a delta Vbe generator circuit. A source of the transistor 110 may beconnected to a supply voltage (e.g., VCC). A node 128 may be formed bycoupling a drain of the transistor 110 with a source of the transistor112 and the transistor 114. The signal VBIAS may be presented at thenode 128. A node 130 may be formed by coupling a gate of the transistor112, a gate and a drain of the transistor 114, and a drain of thetransistor 118. The signal VREF may be presented at the node 130. A node132 may be formed by coupling a drain of the transistor 116, a drain anda gate of the transistor 116, and a gate of the transistor 118. A sourceof the transistor 116 may be coupled to a first terminal of the device120. A second terminal of the device 120 may be connected to a voltagesupply ground potential (e.g., VSS). A source of the transistor 118 maybe coupled to a first terminal of the device 124. A second terminal ofthe device 124 may be coupled to a first terminal of the device 122. Asecond terminal of the device 122 may be connected to the voltage supplyground potential VSS. The first terminals of the devices 120 and 122 maybe connected, in one example, to anodes of the devices 120 and 122. Thesecond terminal of the devices 120 and 122 may be connected, in oneexample, to cathodes of the devices 120 and 122.

[0017] A first input (e.g., a non-inverting input) of the amplifier 126may be coupled to the node 130. A second input (e.g., an invertinginput) of the amplifier 126 may be coupled to the node 132. An output ofthe amplifier 126 may be coupled to a gate of the transistor 110. Theamplifier 126 generally forces a current (e.g., I) through thetransistors 112 and 116 to be the same as a current through thetransistors 114 and 118. The current I may be described by the followingequation 2: $\begin{matrix}{{{{Vbe}\quad 1} = {{{Vbe}\quad 2} + {I \cdot R}}}{I = {\frac{\Delta \quad {Vbe}}{R} = \frac{n \cdot {Vt} \cdot {\ln (B)}}{R}}}} & {{Eq}.\quad 2}\end{matrix}$

[0018] The circuit 104 may comprise a transistor 140, a device 142, atransistor 144, a transistor 146, a transistor 148, and a transistor150. The transistors 140, 148 and 150 may be implemented as one or morePMOS transistors. The transistors 144 and 146 may be implemented as oneor more NMOS transistors. However, other types and polarity transistorsmay be implemented accordingly to meet the design criteria of aparticular application. The device 142 may be implemented as a resistivecircuit. In one example, the device 142 may be implemented as a resistorhaving a predetermined resistance R1.

[0019] The signal VBIAS may be presented to a source of the transistor140. The signal VREF may be presented to a gate of the transistor 140. Adrain of the transistor 140 may be coupled to a first terminal of thedevice 142. The signal NCTR may be presented at the drain of thetransistor 140. A second terminal of the device 142 may be connected tothe voltage supply ground potential VSS. The transistor 140 willgenerally pass a current equal to the current I in response to thesignals VREF and VBIAS. By passing the current I (where I=n*Vt*ln(B)/R,n is the emission coefficient, B is the ratio of diode areas of thedevices 120 and 122, R is a predetermined resistance, and Vt is athermal voltage) through the resistance R1, a voltage may be generated,as shown by the following equation 3: $\begin{matrix}{{NCTR} = {{{I \cdot R}\quad 1} = {{{\frac{n \cdot {Vt} \cdot {\ln (B)}}{R} \cdot R}\quad 1} = {\frac{n \cdot k \cdot {\ln (B)}}{q} \cdot \frac{R\quad 1}{R} \cdot T}}}} & {{Eq}.\quad 3}\end{matrix}$

[0020] When the current I is passed through the device 142, the signalNCTR may be generated having a voltage level equal to ln(B) times Vttimes R1/R. The voltage level of the signal NCTR is generallyproportional to absolute temperature and may be scaled by selecting theratio R1/R.

[0021] The signal NCTR may be presented to a gate of the transistor 144.A source of the transistor 144 and a gate of the transistor 148 may beconnected to the voltage supply ground potential VSS. A drain of thetransistor 144 may be connected to a source of the transistor 146. Agate of the transistor 146 may be connected to the supply voltage VCC. Adrain of the transistor 146 may be connected to a drain of thetransistor 148. A source of the transistor 150 may be connected to thesupply voltage VCC. A node 152 may be formed by connecting a source ofthe transistor 148 with a drain and a gate of the transistor 150. Thesignal PCTR may be presented at the node 152. The signal PCTR may be amirror of the signal NCTR.

[0022] Referring to FIG. 3, a block diagram of a circuit 200 is shownillustrating a voltage controlled oscillator in accordance with apreferred embodiment of the present invention. The circuit 200 may beimplemented, in one example, as a refresh oscillator of a dynamic memorydevice. The circuit 200 may have an input 202 that may receive thesignal PCTR, and an input 204 that may receive the signal NCTR. Thecircuit 200 may comprise a number of inverting amplifier (delay) stages206 a-206 n. In one example, the stages 206 a-206 n may form a currentstarved inverter ring oscillator. The signals PCTR and NCTR may beimplemented as load bias voltages for the delay stages 206 a-206 n. Thecircuit 200 may be configured to generate a signal (e.g., RFRSH) havinga frequency that is proportional to temperature. The signal RFRSH may beused to control a refresh of a memory. For example, the signal RFRSH maybe used to change a refresh rate of the memory in response to atemperature change.

[0023] The circuit 200 may be implemented as a refresh oscillator of adynamic memory device. Since the leakage of the memory cells increasewith increasing temperature, a PTAT voltage-controlled oscillator inaccordance with the present invention may be used to refresh the memorycell more frequently as the temperature increases. The present inventionmay provide temperature dependent refreshing and also may be used in anyapplication requiring a temperature monitor.

[0024] Referring to FIG. 4, a block diagram of a memory device 210 isshown. The memory device 210 is generally shown implemented inaccordance with the present invention. The memory device 210 maycomprise the circuit 100, the circuit 200, and an array of memory cells212. The circuit 100 may be configured to control the refresh circuit200. The refresh circuit 200 may be configured to control refreshoperations on the memory cells of the array 212. For example, Thecircuit 100 may be configured to alter the rate at which the circuit 200refreshes the memory array 212 depending upon temperature.

[0025] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of theinvention. For example, any circuit that generates a current equal to aconstant times Vt/R may be used to generate the PTAT voltage referenceNCTR.

1. A biasing circuit comprising: a first circuit configured to generatea first bias signal and a second bias signal, wherein said second biassignal is defined by a threshold voltage and a first resistance; and asecond circuit configured to generate a third bias signal in response tosaid first and second bias signals and a second resistance, wherein saidthird bias signal has a magnitude that is linearly proportional toabsolute temperature (PTAT) and is configured to vary a refresh rate ofa memory cell in response to changes in temperature.
 2. The circuitaccording to claim 1, wherein said biasing circuit comprises aproportional to temperature voltage generator.
 3. The circuit accordingto claim 1, wherein said first circuit comprises: a first current sourceconfigured to generate a first proportional to absolute temperature(PTAT) current, said first PTAT current defined by a threshold voltage;a second current source configured to generate a second PTAT current inresponse to said first PTAT current, said second PTAT current defined bya threshold voltage, a ratio of diode areas and said first resistance;and a control circuit configured to equalize said first PTAT current andsaid second PTAT current.
 4. The circuit according to claim 1, whereinsaid second circuit comprises: a current source configured to generate aPTAT current that varies linearly with temperature.
 5. The circuitaccording to claim 1, wherein a magnitude of said third bias signal isdetermined by a ratio of said second resistance to said firstresistance.
 6. The circuit according to claim 1, wherein said secondbias signal comprises a bandgap reference voltage.
 7. The circuitaccording to claim 1, wherein said first circuit comprises: a firstcurrent mirror comprising a plurality of PMOS transistors; a secondcurrent mirror comprising a plurality of NMOS transistors, said secondcurrent mirror coupled to said first current mirror; a first diodecoupled directly to said second current mirror; and a second diodecoupled through a resistor to said second current mirror.
 8. The circuitaccording to claim 1, wherein said second circuit further comprises avoltage mirror configured to generate a fourth bias signal in responseto said third bias signal.
 9. The circuit according to claim 3, whereinsaid control circuit comprises: an operational amplifier coupled to saidfirst current source and said second current source and configured toequalize said first PTAT current and said second PTAT current.
 10. Acircuit for generating temperature sensitive biasing of a voltagecontrolled oscillator (VCO) comprising: a first circuit configured togenerate a first bias signal and a second bias signal, wherein saidsecond bias signal is defined by a threshold voltage and a firstresistance; and a second circuit configured to generate one or morethird bias signals in response to said first and second bias signals anda second resistance, wherein said one or more third bias signals have amagnitude that is linearly proportional to absolute temperature (PTAT)and vary a refresh rate of a memory cell with temperature.
 11. Thecircuit according to claim 10, wherein said one or more third biassignals provides load bias voltages to a plurality of delay stages ofsaid voltage controlled oscillator.
 12. The circuit according to claim10, wherein said voltage controlled oscillator is configured to generatea signal having a frequency that varies with temperature.
 13. Thecircuit according to claim 12, wherein said frequency varies linearlywith temperature.
 14. The circuit according to claim 12, wherein saidfrequency variation is proportional to absolute temperature.
 15. Amethod for controlling a refresh rate of a memory using a proportionalto absolute temperature (PTAT) voltage reference comprising the stepsof: (A) generating a first bias signal; (B) generating a second biassignal in response to said first bias signal, wherein said second biassignal is defined by a threshold voltage and a first resistance; and (C)generating a third bias signal in response to said first and second biassignals and a second resistance, wherein said third bias signal has amagnitude that is linearly proportional to absolute temperature(PTAT)and is configured to vary a refresh rate of a memory cell withtemperature.
 16. The method according to claim 15, wherein step Acomprises the sub-steps of: generating a first PTAT current; generatinga second PTAT current; and adjusting said first bias signal to equalizesaid first and second PTAT currents.
 17. The method according to claim15, wherein the step C comprises the sub-steps of: generating a PTATcurrent in response to said first bias signal and said second biassignal; and passing said PTAT current through said second resistance.18. The method according to claim 15, further comprising the step of:presenting said third bias signal to a memory circuit to control arefresh rate.
 19. The method according to claim 18, wherein saidpresenting step comprises the sub-step of: generating a signal having afrequency that increases linearly with temperature.
 20. The methodaccording to claim 19, wherein said increase is proportional to absolutetemperature.